The cost of mailing a letter is going up in January.

In an effort to improve its financial situation, the U.S. Postal Service has made a 5-cent increase on its first-class stamp, which increases the cost of mailing a one-ounce envelope from 50 cents to 55 cents.

The change, which was proposed by the Postal Service's board of governors, was approved by regulators. The 10 percent increase for a one-ounce letter is the biggest increase in postal rates since 1991.